Selective hydrogenation of paracetamol to acetamidocyclohexanone with silylated SiO2 supported Pd-based catalysts

Article abstract of DOI:10.1039/c6ra05479j

A series of catalysts comprising well-distributed Pd nanoparticles incorporated on silylated SiO₂ were fabricated by the wet impregnation method and investigated in the selective hydrogenation of paracetamol to 4-acetamidocyclohexanone. The catalysts calcined at different temperatures were characterized by TG, FT-IR, N₂ physisorption, TPR and XPS. The results showed that organic modification led to a catalyst surface composed of stable Si-(CH₃)₃ species even after calcination at 300 °C. Also, changes occurred in the size and electronic properties of the Pd particles through the different amounts of grafted groups on the SiO₂ support. The mode of adsorption of the paracetamol molecule was influenced by the quite bulky organic groups on the support, resulting in a significant improvement in selectivity towards 4-acetamidocyclohexanone and preventing full hydrogenation to some extent. The best result was obtained on the silylated Pd catalyst calcined at 500 °C, with 64.9% selectivity to keto at the paracetamol conversion of 60.5%, while the non-silylated SiO₂ supported Pd catalyst gave 4-acetamidocyclohexanone selectivity of 29.1% at 53.8% conversion.

Full text of DOI:10.1039/c6ra05479j

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Selective hydrogenation of paracetamol to
acetamidocyclohexanone with silylated SiO2 supported Pd-based
catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Wenjing Songa, Xiuna Liua, Shaoyang Jiangb, Zhou Chena, Weizheng Wenga, I. Rodríguez-Ramosc,
Xiaodong Yi a*, Weiping Fanga
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of catalysts comprising well-distributed Pd nanoparticles incorporated on silylated SiO2 were fabricated by the wet
impregnation method and investigated in the selective hydrogenation of paracetamol to 4-acetamidocyclohexanone. The
catalysts calcined at different temperature were characterized by TG, FT-IR, N2 physisorption, TPR and XPS. The results
showed that organic modification led to a catalyst surface composed of stable Si-(CH3)3 species even after calcination at
300 oC. Also, changes occurred in the size and electronic properties of the Pd particles through the different amount of
grafted groups on the SiO2 support. The mode of adsorption of the paracetamol molecule was influenced by the quite
bulky organic groups on the support, resulting in
a significant improvement in selectivity towards 4-
acetamidocyclohexanone and preventing full hydrogenation to some extent. The best result was obtained on the silylated
Pd catalyst calcined at 500 oC, with 64.9% selectivity to keto at the paracetamol conversion of 60.5%, while the non-
silylated SiO2 supported Pd catalyst gave 4-acetamidocyclohexanone selectivity of 29.1% at 53.8% conversion.
As we all know, both the support and the surface metal
atoms are involved in determining the performance of
catalysts. Traditionally, supported Pd catalysts showed high
1 Introduction
The hydrogenation of substituted phenols to the
corresponding alkyl-substituted cyclohexanones and
selectivity to substituted cyclohexanone in the catalytic
hydrogenation reaction,6, 7 while the hydrogenation of keto
was fast for Ru-catalysts and leaded directly to the 4-
acetamidocyclohexanols.8 Products of hydrogenolytic cleavage
can also be observed with a nickel catalyst.9, 10 There are many
studies dealing with the hydrogenation of paracetamol to
cyclohexanols has generated great interest in the industry and
researchers, due to their application in the subsequent
manufacture of fine chemicals for the fragrance and detergent
industries. The studies are mainly focused on the complete
1
hydrogenation to substituted cyclohexanol, although the
produce acetamidocyclohexanols and the
cis/trans
intermedia product alkylcyclohexanone also plays a significant
role in the industry. Cyclohexanone is well known as an
important intermediate to produce both caprolactam for
stereoselectivity.11-13 However, few studies are concerned with
the partial hydrogenation of paracetamol. It is a challenge to
obtain a high selectivity. Attempts have been made to relate
Nylon
6
and adipic acid for Nylon 66.2 The 4-
12
the selectivity with the nature of the support. Addition of
acetamidocyclohexanone was prepared as an intermediate in
the synthesis of Morphine skeleton.3 Generally, these
cyclohexanones formed in the hydrogenation reaction of
phenols could directly desorb from the catalytic surface or be
further obtained through the oxidation of cyclohexanols or
basic properties to supports has been investigated and
reported to highly improve the partial phenol hydrogenation
reaction.14 And organic modifiers (ligands) on metal-based
heterogeneous catalysts also was reported to help to enhance
the catalytic selectivity to some extent15, 16. With excellent
stability and wide accessibility, silica is widely applied as
support and the silanized organic groups on the surface can
5
cyclohexane.4, But the latter route involves two steps,
focusing the investigation on the former facile route for one-
pot cyclohexanone formation.
affect the anchor of precious metal and the catalytic
17
performance.
In the same way, the hydrophilic oxygen-
containing groups on the carbon support improve the
solubility and act as the site for further chemical derivatization
(amidation or esterification).18 Bachiller-Baeza et al., 8, 12 after
investigating the influence of different supports and
promoters on the stereoselective hydrogenation of
paracetamol to 4-acetamidocyclohexanol, found that the OH
groups on carbon-supported catalysts diminished the activity
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500 oC was named Pd/Si-500.
in the reaction explaining the catalytic performance as a
function of the acid-base properties of the catalyst surface.
19
2.3 Catalyst characterization
Thermogravimetric analysis (TGA) of the catalysts was carried
out on TGA Q600-SDT apparatus. The sample powders were
heated under an air flow of 100 mL·min-1 from 50 to 800 oC at
heating rate of 10 oC·min-1.
Kim et al. increased the activity of SBA-15 supported Co
catalysts for the FT synthesis via a hydrophobic silylation of the
support, and these groups on SBA-15 enhanced the reducibility
of the cobalt oxide species due to the weaker cobalt oxide-
20
support interaction. Besides, Tan et al.
reported the
Nitrogen adsorption isotherms were recorded at the
temperature of liquid nitrogen using a Micromeritics ASAP
3020 apparatus. Before the analysis, the samples were
outgassed at 110 oC. Specific areas were calculated by applying
the Brunauer-Emmett-Teller (BET) method to portions of the
isotherms within the 0.05<P/P0<0.30 relative pressure range.
The total pore volume was calculated from the amount of
vapor adsorbed at a relative pressure close to unity assuming
that the pores are filled with the adsorbate in the liquid state.
FT-IR spectra of pyridine adsorption were recorded using
a Thermo Nicolet Nexus spectrometer equipped with a liquid-
nitrogen-cooled MCT detector. The samples were pressed into
modification of Al2O3 by a silylant agent which introduces
amino functional groups at the surface. The -NH2 group offers
ideal host for uniform distribution of Ru metal to improve the
dispersion of Ru nanoparticles and favors the formation of
electron-rich Ru sites by coordination with the amino ligands.
Besides, the different capacity of mpg-C3N4 (polymeric
mesoporous graphitic carbon nitride) support to adsorbs
phenol and cyclohexanone was suggested to enhance the
semihydrogenation rate of phenol on Pd-mpg-C3N4 catalysts.21
In our present work, we attempt to tune the surface
chemical composition of heterogeneous metal catalysts to
optimize their catalytic performances. The silylant agent used
was 3-aminopropyltriethoxysilane introducing amino groups,
and modifying the OH groups at the support surface, which
provides sites to anchor the Pd nanoparticles and changes the
adsorption way of reactant and products during the reaction.
The amount of this silylant agent grafted on the surface of SiO2
was varied by application of different calcination processes.
The selective hydrogenation of paracetamol to aromatic
ketones was studied over the different silylated SiO2 supported
Pd catalysts. The correlation of the catalytic behavior in the
reaction with the coverage of SiO2 support by the silylant
agent and the availability of H atoms on Pd nanoparticles
simultaneously assessed was carried out.
o
self-supporting wafers and treated in H2 at 300 C in an IR cell
o
for 1 h followed by evacuation at 300 C for 5 min to remove
the gas phase H2. After cooling to 100 oC, the samples were
exposed to pyridine vapor for 10 min. Then the spectra were
recorded after evacuation at high temperatures. The IR spectra
were recorded in the spectral range 1700 to 1400 cm-1 with 32
scans and at a resolution of 4 cm-1.
Transmission electron microscope (TEM) experiments
were performed using FEI Tecnai 30 high resolution
transmission electron microscope (Philips Analytical) operated
at an accelerating voltage of 300 kV. All the samples were
o
reduced with H2 at 300 C and prepared by depositing a drop
of diluted suspensions in ethanol on a carbon film coated
copper grid before TEM experiments.
2 Experimental
2.1 Silylation process
The silylated SiO2 was prepared by impregnation of silica gel
Temperature programmed reduction (TPR) analyses were
carried out with the GC-TPR apparatus. The samples (50 mg
each) were pretreated in a 5% O2/Ar flow of 30 ml·min-1 at 300
with the (3-aminopropyl) triethoxysilane (APTES) solution in
toluene. 10 g silica gel supplied by Sigma-Aldrich (Davisil,
Grade 636, 435 m2/g, 35-60 mesh, pore size of 6 nm) was first
o
oC for 1 h and then cooled to about 11 C with the circulating
（
50 ml·min-1) was switched and
o
cooling water. Next 5% H2/Ar
treated with 100 mL 20wt% HNO3 at 30 C for 24 h, followed
the sample was heated to 500 C at a rate of 10 C·min-1 until
the flat baseline was obtained. The H2 consumption of the
catalyst reduction was determined by a thermal conductivity
detector (TCD).
o
o
by washing thoroughly with deionized water until pH=7 and
dried overnight at 110 oC before use. The silica sample was
then impregnated with 5%vol. APTES dissolved in toluene
solution.22 Aliquots of 0.6 g support were suspended in 10 ml
toluene with 0.5 ml APTES at room temperature overnight.
The silylated support was filtered, washed with ethanol, and
then dried at 120 oC for about 4 h.
X-ray photoelectron spectroscopy (XPS) measurements
were made with a Qtac100 spectrometer using the incident
radiation Al Ka of 1486.6 eV at 250 W, 20 mA, and 50 eV of
pass energy. This system has a prechamber for samples
introduction and in situ heat treatment. The samples were
2.2 Catalyst preparation
3wt% Pd catalysts were prepared by the wet impregnation
o
reduced at 300 C in flow of H2 before transferred. The Si 2p
method, using 0.02 M H2PdCl4 (the mole ratio of PdCl2 and HCl
was 1/2) as the palladium precursor. The silylated SiO2 was
then impregnated with the above Pd solutions at room
temperature for 24 h. After drying at 120 oC, the catalysts were
calcined in air at different temperatures for 2 h. The catalyst
samples were designated as Pd/Sily-x, where x represents the
calcination temperature. For comparison, the SiO2 treated by
HNO3 was also used as the support, and this catalyst heated at
binding energy was taken as a charge reference, fixed at 103.3
eV. To quantify the Pd species contents, the obtained XPS
spectra were fitted using XPSPEAK41 software. The atomic
ratio estimations were done according the peak areas after the
background subtraction and corrected relative to the
corresponding atomic sensitivity factors.
2.4 Apparatus and procedure for activity tests
2 | J. Name., 2012, 00, 1-3
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The hydrogenation of paracetamol was carried out under 5
MPa H2 pressure in a Parr 4848 autoclave fitted with a
magnetically-driven stirrer. After being reduced under flowing
hydrogen at 300 oC for 2 h, the Pd catalyst (0.1 g) was
transferred in ethanol to the stainless steel reactor that
contained
a
solution of paracetamol (0.214
g
4-
acetamidophenol, AR: 99.0%, from Aladdin) in ethanol (weight
ratio of substrate and catalyst was 71). The autoclave was
o
closed and purged in He several times, heated to 170 C with
He, then pressurized to 5 MPa with H2. The analysis was
performed by gas chromatograph equipped with a flame
o
ionization detector operating at 200 C and a capillary column
OV-17 (30 m
×0.32 mm). The injector temperature was set to
o
200 C, the following temperature program method was used
for column: 150 oC for 5 min and then heated to 250 oC with a
ramp rate of 10 oC·min-1 and kept at 250 oC for another 7 min.
The main products include 4-acetamidocyclohexanols and 4-
acetamidocyclohexanone, and the other product is the N-
cyclohexylacetamide. The normalization method was used to
determine the conversion and selectivity.
3 Results and discussion
3.1 Characterization of the catalysts
The thermal decomposition process of SiO2 supported Pd
catalyst was investigated by thermal gravimetry and shown in
Fig. 1. The weight loss below 200 oC was related to desorption
of physical adsorbed water on SiO2. Comparison of Fig. 1(a), (b)
and (c) suggest the weight loss of Pd/Sily-110 catalyst
comprised the decomposition of both the silylating agent, and
OH groups remaining on the silica surface. A strong exothermic
peak was presented around 300 oC with the silylated SiO2,
which indicated the decomposition of (3-aminopropyl)
triethoxysilane. Comparison of the Fig. 1(b) and (c) indicates
the decomposition of OH groups proceed between 300 and
600 oC. The thermogravimetric analysis of Pd/Sily catalysts
showed that the Pd/Sily-110 catalyst was covered by the
silylant agent while a few APTES existed on the Pd/Sily-500,
compared with the full decomposition happened under 800 oC
calcination in air.
The IR spectroscopy was evaluated to gain insight into the
structure of silylated SiO2. Methyl groups were incorporated
by silylation, as shown in the Fig. 2. The bands at 2963 and
2908 cm-1 represented the stretching vibration of C-H bonds in
CH3 and CH2 groups respectively whereas the bands detected
at 3366 and 3277 cm-1 were corresponded to primary and
secondary amine groups.20, 23 It indicated the incorporation of
silylant agent on the Pd/Sily catalyst. And the amount of these
groups showed a diminished trend with respect to calcination
temperature: Pd/Sily-110>Pd/Sily-500>Pd/Sily-800. This result
strongly coincided with that obtained from TGA.
FT-IR spectra of pyridine adsorbed on reduced catalysts
were shown in Fig. 3. All the samples only showed the
characteristic absorption bands at 1450 cm-1 of pyridine
adsorbed on Lewis acid sites and no bands (1540 cm-1) due to
Fig. 1 TG curves and DSC curves of (a) silylated SiO2; (b) Pd/Si
catalyst; (c) Pd/Sily-110; (d) Pd/Sily-300; (e) Pd/Sily-400; (f)
Pd/Sily-500; (g) Pd/Sily-800.
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diameter were smaller than those of non-silylated SiO2 supported
Pd catalyst. This finding supports the presence of surface organic
residues inside the porosity of SiO2.
Much more information about the variation in Pd particle size
for the silylated SiO2 supported palladium catalysts during the
different calcination processes could be obtained from TEM
experiment, as shown in Fig. 4. The dark spots in the images, which
represented the Pd nanoparticles of roughly spherical shape, were
well distributed on the supports. The particle size distribution
Fig. 2 Evolution of the IR spectra of the Pd catalyst as a
function of calcination temperature: (a) Pd/Sily-110; (b)
Pd/Sily-500; (c) Pd/Sily-800
Fig. 3 FT-IR spectra of pyridine adsorbed and desorbed on (a)
Pd/Si-500; (b) Pd/Sily-300; (c) Pd/Sily-400; (d) Pd/Sily-500; (e)
Pd/Sily-800 catalysts at 150 oC.
24
pyridine adsorbed on the Brönsted acid sites were observed. The
intensity of absorption bands for Lewis acid sites (1450 cm-1) on the
silylated Pd samples were increased with calcination temperature.
These results suggested that the Lewis acidity of catalysts were
decreased on silylated Pd samples compared to Pd/Si-500.
Surface areas calculated by application of the BET method to
the isotherms of N2 adsorption are given in Table 1. A gradual
increase of the surface area values as the temperature of thermal
treatment increases was observed in the samples. This suggests the
existence of organic groups coordinated with the OH on the SiO2
which decreased the surface area. Furthermore the surface area
value of Pd/Sily-800 was lower due to the effect of the structure of
SiO2 after calcination at 800 oC. The pore volume and average pore
Fig. 4 TEM micrographs for the reduced Pd/Sily catalysts. (a)
Pd/Si-500; (b) Pd/Sily-300; (c) Pd/Sily-400; (d) Pd/Sily-500; (e)
Pd/Sily-800.
Table 1 The properities of Pd catalysts
Pd 3d5/2 BE(eV)
Volume
/cm3·g
0.80
Pore Size
d/nm
(TEM)
6.1
Pd/Si
(XPS)
0.01
0.06
0.03
0.04
0.01
Cat
SBET/ m2·g
Pd2+
Pd0
/nm
6.2
5.6
5.6
6.4
5.6
Pd/Si-500
Pd/Sily-300
Pd/Sily-400
Pd/Sily-500
Pd/Sily-800
399
374
377
383
379
336.1
336.1
336.1
336.1
336.1
334.9
334.8
334.9
334.9
335.0
0.67
5.2
0.63
5.3
0.68
4.4
0.67
5.4
4 | J. Name., 2012, 00, 1-3
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two positive peaks (a large one at 32 oC and a small one at
65oC) were due to two kinds of Pd oxide, amorphous and
27, 28
crystalline PdO (PdaO and PdcO).
The reduced Pd was
highly susceptible to atmospheric oxygen, leading to the
amorphous Pd oxide layer, and the contribution from the Pd-
Pd coordination shell of the Pd oxide layer was weak relative
to the Pd-O shell. On the other hand, all the catalysts exhibited
a positive reduction peak, with the maximum at ~120 oC, after
the hydride phase decomposition, which could be ascribed to
29
the reduction of two-dimensional surface complex [PdO]sc.
The organic modification decreased the interaction between
PdO and Si-OH groups, resulting in the redistribution of Pd and
the formation of [PdO]sc, while at the higher treatment
temperature, the [PdO]sc evolves to PdaO and PdcO species. As
seen from the tendency of Pd/Sily-300, the PdaO species might
be reduced to metal below 13 oC. And the TCD signal for
palladium hydride also enhanced by the increase of amount of
PdaO, as treatment temperature increased, resulting the less
silicified organic groups on the support, the less disordered
PdO clusters formed and the PdO species shifted from
amorphous to crystalline.
Fig. 5 H2-TPR profiles of the (a) Pd/Si-500; (b) Pd/Sily-300; (c)
Pd/Sily-400; (d) Pd/Sily-500; (e) Pd/Sily-800 samples.
obtained by measuring more than 300 individual particles gave
a mean particle size on the sample. Pd particles with size of
about 4-7nm were observed in all the catalysts. And Pd/Sily-
500 exhibited a smaller particle size of about 4.4nm compared
with the non-silylated SiO2 supported Pd catalyst with 6.1nm.
Upon the APTES modification, palladium precursors were
easily adsorbed, and Pd nanoparticles were uniformly
embedded in SiO2.25 During the preparation of catalyst, the
amine function reacts with the Pd precursor (H2PdCl4) in an
simple base-acid reaction, favoring by this way the dispersion
of Pd on the support.
Besides the hydrogen consumption peak, the negative
peak for the palladium hydride (β-PdH) decomposition was
also present on the supported Pd catalyst, and the hydride
formation was a bulk phenomenon.30,
31
The temperature
and the amount for the decomposition was reflected by the
stability of the hydride phase32 and the particle size,29
respectively. The higher decomposition temperature of the β-
PdH indicated the larger crystallites size. This result was in
good agreement with that reported by Nag.32 It was suggested
that as the order in the Pd lattice increased, the electronic
nature of the palladium changed, leading the increase of the
binding force holding the hydrogen in the lattice. Besides,
Bonarowska33 studied the kind of decomposition peaks,
ascribing the 90 oC peak to hydrogen evolution from a massive
palladium hydride, and the low-temperature peak (70 oC) to
the more finely dispersed Pd. The smaller formation of the β-
hydride phase on the silylated catalysts treated at 300 oC or
400 oC was ascribed to the absence of PdaO in the catalyst, due
to more formation of disorder PdO on the silylated SiO2.
The interactions between the active phase and the support
were examined by temperature-programmed reduction (TPR).
This also reflected the reduction behavior of the Pd species of
each sample. Some differences of the reduction peaks on Pd
catalysts were shown in Fig. 5. The reduction peaks could be
assigned to several processes and the reduction temperature
26
o
changed based on the support. The peaks around 40 C can
be attributed to the reactions (1), (2) for hydrogen
consumption by the palladium oxide formed during the
calcination, while the negative reduction peak around 80 oC
was related to the hydride phase decomposition as shown by
the reaction (3)26
To obtain information about the valence state of metal on
the catalyst surface, we carried out a XPS study. The XPS
spectra are shown in Fig. 6. The binding energies were
compiled in Table 1. The Pd 3d5/2 peaks line of all the catalysts
were split in two components, one at 334.6 eV indicated that
(1+1/2x)+PdaO
1.5H2+PdcO PdcH+H2O
PdaHx Pda+1/2xH2
→
PdaHx+H2O (1)
→
(2)
(3)
→
It has been reported that the reduction temperature for the
TiO2 supported palladium was below 0oC and for the
uncalcined Pd/Al2O3 the reduction took place between 80 oC
34
palladium was essentially as metallic Pd, while the peaks
placed at 336.5~336.8 eV were due to the PdO.35, 36 Meanwhile,
a decrease of Pd/Si atom ratios with higher calcination
o
and 230 C. In our case for the SiO2 supported Pd catalyst, the
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temperature was observed, and a richer Pd was anchored on
the silylated Pd sample.
3.2 Catalytic activity
Considering the results previously reported,11 the
hydrogenation of paracetamol could follow the consecutive
reaction pathways (Scheme 1). The benzene ring of
paracetamol is first partially hydrogenated to an enol, which is
unstable and isomerizes rapidly to form keto; keto could then
be further hydrogenated to form 4-acetamidocyclohexanol.
And in the hydrogenation process, some hydrolysis products
(N-cyclohexylacetamide) may be obtained. The most
interesting feature of the present work was the marked
difference in the selectivity to 4-acetamidocyclohexanone
obtained under the present experimental conditions with the
modified catalysts.
Scheme
1
Reaction pathways in the hydrogenation of
paracetamol.
Table 2 Catalytic performance on the hydrogenation of
paracetamol at 170 oC, 5 MPa H2 in 20 min.
Selectivity (%)a
Conv.
Keto
Yield
15.6
28.2
35.2
39.3
23.1
Cat.
(%)
Sk
SH
SN
Pd/Si-500
Pd/Sily-300
Pd/Sily-400
Pd/Sily-500
Pd/Sily-800
53.8
39.5
51.9
60.5
49.7
29.1 69.7 1.2
71.4 26.5 4.1
67.9 29.7 2.4
64.9 32.4 2.7
46.5 52.2 1.3
The results of the hydrogenation of 4-acetamidophenol
are given at Table 2. The activity evolution of two catalysts as a
function of time in the reaction is shown in Fig. 7. 4-
acetamidocyclohexanone was the primary product observed,
as expected from the nature of the reaction. The selectivity to
the intermediate 4-acetamidocyclohexanone decreased with
the increasing conversion. This was in agreement with phenol
a SH, SK, and SN indicate the selectivity to 4-acetamidocyclohexanol,
4-acetamidocyclohexanone
respectively.
and
N-cyclohexylacetamide,
hydrogenation which followed
a
consecutive reaction
pathway.39The concentration of keto increased with time and
decreased after reaching a maximum in both cases. After 4 h
reaction, the paracetamol was almost totally converted into
acetamidocyclohexanol over Pd/Si-500 catalyst. The deep
hydrogenation product is widely used as a bronchosecretolitic
agent for the prevention and treatment of asthma, acute and
chronic bronchitis.40 But the reaction on Pd/Sily-500 remained
at the step of 4-acetamidocyclohexanone with the selectivity
of 64.9%. It is deduced that the silylated Pd catalyst is active
for the first step but is not active for the second step under the
experimental conditions.
temperature increases less silylating agent was kept in favor of
enhanced activity. The conversion of paracetamol increased
with the calcination temperature, passing through a maximum
(60.5%) for the catalyst Pd/Sily-500 and dropping for sample
treated at higher temperature. The marked difference was the
product
distribution.
64.9%
selectivity
to
4-
acetamidocyclohexanone was obtained on Pd/Sily-500
compared with 29.1% for Pd/Si-500 catalysts. The specific
activity of catalyst did not seem to be greatly affected by the
particle size considering the narrow size range of the Pd
particles (4-7nm). This was in accordance to the results
previously reported in the hydrogenation of phenol.41, 42
The conversion of paracetamol increased with the
calcination temperature, passing through a maximum (60.5%)
for the catalyst Pd/Sily-500 and dropping for the Pd/Sily-800.
This can be ascribed to the adsorption H atom on the Pd metal.
From TPR profiles, the palladium hydride species decompose
at lower temperature on the Pd/Sily-500, indicating the
weaker absorption of H2 on the Pd. The easier desorption of
hydrogen facilitate the activity. The hydrogen atom is difficult
to desorb from the metal and participate to the hydrogenation
reaction, leading to the lower conversion. The silylation
process favors the formation of hydride phase on the Pd
catalyst. After calcination at higher temperature, the organic
groups are reduced on the SiO2 surface and the absorption of
H atom on Pd metal is stronger. The stable presence of PdH on
the Pd/Sily-800 results in the lower amount of H2 desorption
on the Pd, so the conversion was only 49.7% for the Pd/Sily-
800 catalyst.
As shown in the characterization measurements, at the
lower calcination temperature remained the higher amount of
functional groups incorporated on SiO2 spheres, leading to the
lower conversion in the reaction. As the calcination
Fig. 6 Pd 3d XPS spectra of catalysts (a) Pd/Si-500 and (b)
Pd/Sily-300; (c) Pd/Sily-400; (d) Pd/Sily-500; (e) Pd/Sily-800
6 | J. Name., 2012, 00, 1-3
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As the selectivity of catalyst is concerned, the structure of
the catalyst and the electronic surrounding of metal both
played an important role in the adsorption of the reactant and
active hydrogen respectively. The TG and FTIR spectra of
pyridine adsorption results showed the remained silylating
agent decreased the intensity of Lewis acid on the catalysts.
And the electronic palladium surrounding was modified by the
organic groups on the support. It favored the back donation of
electrons to the π* (anti-bonding) of aromatic ring in
chemisorbed phenol, making the aromatic ring activated
towards hydrogenation.43 On the other hand, in the
hydrogenation of phenol, the process selectivity was
interpreted in terms of reactant/catalyst interactions where
phenol adsorption was viewed as occurring via aromatic π-
electron system.44,
45
The strong interaction between the
aromatic ring and the support favors the formation of
cyclohexanol with a coplanar adsorption. On the contrary,
phenol can be chemisorbed in a nonplanar form, which
facilitates the production of cyclohexanone.46, 47 As depicted in
Fig. 8, two different adsorption modes adopted by
paracetamol on the surface of catalyst via the oxygen atom of
the hydroxyl group in the form of phenolate, included the non-
planar and the co-planar. The quite bulky sily groups present
on the surface of silylated catalysts could hinder the planar
adsorption of the paracetamol molecule on the Pd
nanoparticles. The planar or non-planar adsorption of
paracetamol depends of the amounts of silylated groups on Fig. 7 Concentration profiles of paracetamol and the products
the catalyst surface. Actually, the selectivity to 4- as the function of time on Pd catalysts under reaction
conditions: T=170 oC, PH2= 5 MPa, Wcat. = 0.1g, substrate/Pd=71
acetamidocyclohexanone decreases when the calcination
o
temperature increases from 300 to 800 C, as the amount of wt%, solvent: ethanol. (a) Pd/Si-500; (b) Pd/Sily-500.
silylated groups decrease. When the sample was treated at the
lower temperature, the APTES anchored on the support
impedes the planar adsorption of the paracetamol via O-Pd
bonding on the catalyst surface due to steric hindrance, thus
preventing them from further hydrogenation.48 The absence of
sily groups on Pd/Sily-800 catalyst results in more co-planar
aromatic ring adsorption and producing less 4-
acetamidocyclohexanone with the 46.5% selectivity. The non-
silylated SiO2 supported Pd catalyst favors the co-planar
absorption of reactant, obtaining the 4-acetamidocyclohexanol
and carrying out full hydrogenation.
Fig. 8 The possible structure and hydrogenation route for
Pd/SiO2 catalyst.
4 Conclusions
On the basis of the results reported in this paper, it can be
concluded that the catalytic behavior of Pd/silylated SiO2
towards the selective hydrogenation of paracetamol to
acetamidocyclohexanone is strongly affected by the steric
hindrance of bulky sily groups on the support and the electron
surroundings of metal active sites (Pd). The surface groups,
which can act as the centers to interact with the precursor, are
influenced by the silylating agent. Meanwhile, the reducing
ability of the catalysts and the stability of the hydride phase
could be tuned by different calcination temperatures. The
decomposition of Pd-H species influence the hydrogenation
activity and positive effect on yield of keto has been identified
as due to the steric hindrances of the sily groups on the
support. Compared with the Pd/Si-500 without silylation
process, the silylated SiO2 produced smaller particles of Pd
(Pd/Sily-500) and gave the better selectivity to 4-
acetamidocyclohexanone (64.9 %) at 60.5 % conversion in 20
min.
Acknowledgements
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ARTICLE
Journal Name
27 J. Keating, G. Sankar, T. I. Hyde, S. Kohara and K. Ohara,
Phys. Chem. Chem. Phys., 2013, 15, 8555-8565.
This work is supported by the National Natural Science
Foundation of China (21373168, 21473143) and the Science
and Technology Foundation of Guangxi (14125008-2-10).
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Selective hydrogenation of paracetamol to acetamidocyclohexanone with silylated SiO2
supported Pd-based catalysts
The decomposition of Pd-H species influence the hydrogenation activity and positive effect on yield of
keto has been identified as due to the steric hindrances of the sily groups on the support.